Flash butt welding is widely used as a welding method of steel. This method has beneficial features of automation feasibility, highly stable quality, short welding time, and the like.
The principles of the flash butt welding method will be described using FIGS. 1A to 1D.
Firstly, as shown in FIG. 1A, a voltage is applied to each of a pair of rail steels 1A and 1B, which are materials to be welded that are disposed opposite to each other, through electrodes 2 from a power supply 3. Simultaneously, the rail steel 1A is moved in the arrow 4 direction so that both end surfaces, which are surfaces to be welded of the rail steels 1A and 1B, are gradually brought close to each other. Then, a short-circuit current locally flows, and the end surfaces are abruptly heated and finally melted by resistance heating. As a result, the pair of rail steels 1A and 1B is bridged with molten metal. In the bridged portion, as show in FIG. 1B, an arc is generated, and a part of the molten metal is scattered (flashing). Furthermore, the end surfaces are heated by resistance heating and arc heating together with the occurrence of flashing, and the two are continuously repeated. This is termed a flashing process.
In addition, the process as shown in FIG. 1C is a process for carrying out heat input into the entire end surfaces of the rail steels 1A and 1B within a short time in the flashing process, and is termed a preheating process. In the preheating process, firstly, a large electric current is made to flow for a certain time in a state in which the pair of the rail steels 1A and 1B are forcibly brought into contact with each other, the vicinities of the end surfaces are heated by resistance heating, and then the pair of the rail steels 1A and 1B are pulled apart. These processes are repeated several times.
Since the preheating process effectively carries out heat input into the welding surface, and produces an effect of shortening the welding time, a welding method is employed in which the preheating process is combined with the flashing process. In addition, since flash butt welding is carried out in the atmosphere, a large amount of oxide is generated in the formed welded metal portion.
A velocity at which the pair of the rail steels are brought close to each other in the flashing process is termed a flashing velocity. In addition, the erosion amount of a molten, substance removed when the rail steels are brought close, and the molten metal is scattered in the flashing process is termed flash-off distance. If the flashing velocity becomes excessive while the heat input into the surfaces to be welded is not sufficient, a phenomenon termed freezing occurs. In freezing, the contact area is abruptly increased without occurrence of an arc or scattering of the molten metal, a large current flows, and flashing does not occur continuously. Since the freezing generates oxides, which deteriorates the bending performance, it is necessary to avoid the freezing as much as possible. In order to prevent the flashing from occurring, the balance between appropriate heat input into the surfaces to be welded, and the flashing velocity is important.
After the entire surfaces to be welded are eventually melted in the flashing process, as shown in FIG. 1D, the surfaces to be welded of the rail steels 1A and 1B are swiftly held together by a large welding pressure, the majority of the molten metal in the welded surfaces is removed outside, and portions behind the welded surfaces, which are heated to a high temperature, are pressed and deformed, thereby forming a joining portion. This is termed an upset process.
At this time, since the oxide generated during welding is miniaturized and dispersed while being exhausted, it is possible to decrease a possibility of the oxide remaining on the joined surfaces as a defect that inhibits the bending performance.
The oxide exhausted outside the joined surfaces in the upset process (bead portion) is removed by hot shearing or the like in the post processes.
In the above flash butt welding, since the respective welding processes are automated, the total welding time of the entire welding processes is short, 1.5 minutes to 4 minutes, resulting in a high welding efficiency. Therefore, in the field of rail production, flash welding is frequently employed as a factory welding method. In addition, the flash welding allows welding apparatuses to be compact, and is used for on-site welding on rail tracks.
As described above, flash butt welding is a technique that joins a pair of steel materials by heating and melting a pair of end surfaces of the steel materials, and then holding the end surfaces together with pressure. Here, the steel material welded in flash butt welding undergoes a temperature increase process, in which the steel material is heated from room temperature to a melting point, and a subsequent cooling process. As a result, the metallic structure thereof transforms. As such, an area in which the structure or mechanical properties, such as hardness, of the material to be welded are changed due to welding is termed a heat-affected zone (HAZ).
In determining the range of the HAZ, measurement of the area in which the mechanical properties are changed consumes time and efforts, such as hardness measurement. Therefore, it is common to employ relatively easy micro or macro observations, and the ranges which can be differentiated from the base material by such observations are designated as the HAZ (Non-patent document 1). In the present specification, an area which can be differentiated from the base material by the micro or macro-observation, as described later, will be termed a HAZ.
Rail steel made of hypereutectoid steel with a high carbon content contains 0.85% to 1.20% of C, and exhibits a pearlite structure. The pearlite structure exhibits a lamellar structure in which phases are alternately and densely overlapped: one of the phases being a pure iron phase which includes almost no carbon and is termed ferrite; and another being a layer of iron carbide (Fe3C) termed cementite. In the process that generates pearlite, the transformation energy is converted to the interface energy of ferrite and cementite, and therefore the lamellar structure is formed.
Here, the structure transformation of rail steel exhibiting the pearlite structure during the temperature increase process is as follows.
(1) From room temperature to 500° C., the pearlite structure does not change.
(2) When the temperature exceeds 550° C., a change in the structure to reduce the interface energy of the lamellar structure, that is, decoupling and spheroidizing of the cementite begin. The spheroidizing of the cementite progresses as the temperature increases.
(3) The transformation of the pearlite structure to an austenite structure begins around the Ac1 transformation point of 720° C. As a result, a temperature region is present in which three phases of ferrite, spherodized cementite (spherical cementite), and austenite coexist in metal.
(4) When the temperature is further increased, either phase of ferrite or cementite is lost, and the three-phase structure turns into a two-phase structure of austenite and spherical cementite, or austenite and ferrite.
(5) When the temperature is further increased, a single-phase structure of austenite is formed.
(6) When the temperature is further increased and exceeds the melting point (solidus temperature), a molten phase is formed in the austenite structure.
(7) When the temperature is further increased, the pearlite structure is completely melted.
In flash butt welding, the peak temperature varies with the distance from the welded surface. That is, the peak temperature reaches higher than the melting point at the welded surface, but remains at room temperature in a sufficiently far portion from the welded surface. In summary, in the HAZ in rail steel exhibiting the pearlite structure, any of the structure transformations (1) to (7) occurs depending on the peak temperature. Specifically, the HAZ is divided into (1) a pearlite area (no change), (2) a spherical cementite area, (3) a three-phase area in which austenite, ferrite, and spherical cementite coexist, (4) a two-phase area of austenite and ferrite or austenite and spherical cementite, (5) an austenite single-phase area, (6) an area in which a mixture of an austenite phase and a molten phase is present, and (7) a completely molten area from the sufficiently far portion from the welded surface to the welded surface.
In the structures transformed in the temperature increase process, additional structure transformations occur respectively due to cooling, depending on decreases in the temperature, when the heating process of welding is finished, and a hardness distribution is formed in accordance with the structure transformations. The hardness distributions vary with the structures and the components, but the case of high-strength rail steel for a heavy load railway having a base material hardness Hv of 420 will be described below as an example.
(1) The pearlite area (the portion in which no structure transformation occurs in the temperature increase process) remains unchanged even after the cooling.
(2) In the spherical cementite area, the spherical cementite is cooled with no change, and exhibits a spherical structure even at room temperature. The hardness of the spherical cementite structure is low and approximately 300 Hv. That is, in the temperature increase process, spheroidizing of the cementite progresses as the peak temperature increases, and therefore the amount of the spherical cementite is increased toward the welded surface. Therefore, the spherical cementite area becomes more softened toward the welded surface in the cooling process.
(3) In the three-phase area in which austenite, ferrite, and spherical cementite coexist, as the temperature decreases, the austenite is transformed into pearlite, and the spherical cementite is cooled with no change to room temperature. Since the fraction of the austenite phase is increased as the peak temperature increases, and the fraction that turns into pearlite after the cooling is increased, the hardness is more restored toward the welded surface. The hardness of the spherical cementite structure is low and approximately 300 Hv.
(4) In the two-phase area of ferrite and austenite or austenite and cementite, the austenite is transformed into a pearlite structure during the cooling. Since the fraction of the austenite phase is increased as the peak temperature increases, and the fraction that turns into pearlite after the cooling is increased, the hardness is more restored toward the welded surface.
(5) In the austenite single-phase area, the austenite is transformed into a pearlite structure. The hardness of the area becomes almost constant.
(6) In the austenite area in which the molten phase is present, the liquid phase is firstly solidified into austenite, thus turns into an austenite single phase, and then is transformed into a pearlite structure. The hardness of the area becomes almost constant.
(7) The molten area is firstly solidified into an austenite single phase, and then transformed into a pearlite structure. The hardness of the area becomes almost constant.
As such, regardless of the temperature regions from which portions are cooled, every portion turns into a pearlite structure in which the ferrite and the cementite eventually form a lamellar structure. However, the areas (2) and (3) include the spherical cementite structure, thus are softened, and the hardness is changed depending on the fraction of the spherical cementite structure.
Therefore, a softened portion having a lowered hardness is generated in the welded portion of rail steel. When the softened portion is long in the longitudinal direction of a rail, and, furthermore, the hardness is significantly lowered, uneven wear progresses in the softened portion due to train wheels passing on the rail head portion, and a variety of problems occur.
Patent document 2 shows a hardness distribution in a welded portion in a joint of pearlite steel that is flash-butt-welded, and, in the document, the HAZ width is approximately 42 mm, and the softened width is approximately 25 mm to 30 mm.
Meanwhile, Patent Document 3 describes that, in railway rails, when the softened width is narrower than the contact area between train wheels and the rail, uneven wear does not easily occur, and, furthermore, since the contact area between train wheels and the rail is considered to be approximately 15 mm, the softened width having a hardness lower than that of the base material by 50 or more is desirably 15 mm or less.
FIG. 2A shows a macro cross-section in the longitudinal direction of a welded portion in a joint formed by welding hypereutectoid rail steel with a high carbon content by a flash butt welding method of the related art. FIG. 2B shows the hardness distribution from the rail surface layer to a depth of 5 mm in the vicinity of the welded portion in the joint as shown in FIG. 2A.
In addition, in the welding, a flash welder having an AC power supply, a transformer capacity of 240 kVA, and an upset load of 70 kN was used, preheating was carried out 7 times, the total time of an initial flashing process as shown in FIG. 3, which will be described below, and a former flashing process was set to 120 seconds, the latter flashing velocity in the latter flashing process was set to 0.5 mm/sec, and the latter flash-off distance was set to 3 mm.
As is clear from FIGS. 2A and 2B, the HAZ boundaries, which are determined on the macro cross-section, are located closer to the central side of the welded portion than ranges in which the hardness is changed, and are located slightly outside the location at which the hardness is most decreased. In FIG. 2B, the HAZ width is 35 mm, and the softened width is 19 mm, which shows that there is a concern regarding the above uneven wear.
Meanwhile, the softened width refers to a range in which the hardness falls below that of the base material. Since the hardness of the base material is also slightly inconsistent in actual cases, the softened width is set to a range in which the hardness falls below (the average value of the base material hardness−3×standard deviation).
However, since the equivalent hardness of the base material can be restored at the center of the welded portion when a thermal treatment is carried out as shown in Patent Document 1, basically, the center of the welded portion is not included in the softened portion. Meanwhile, in a case in which a thermal treatment is not carried out, or the effect of a thermal treatment is small, and the hardness at the center of the welded portion does not reach the base material hardness, an auxiliary line is drawn toward the central side of the welded portion in the hardness distribution, and a range determined by the intersection of the line with the (the average value of the base material hardness−3×standard deviation) forms the softened width. In FIG. 2B, the softened width was obtained by this method, and was 19 mm.
Regarding the above problem of softening in the flash-butt-welded portion, the following technique is proposed.
Patent Document 3 discloses a technique in which rails are flash-butt-welded with a dolly block mounted thereon, and thus the head portion of the rail is cooled by the dolly block during welding. The contact range between the dolly block and the rail includes at least the head top surface of the rail in the cross-section of the rail, and the length of the contact range in the rail axis direction on the head top surface is 15 mm or more. The thickness of a portion in which the dolly block and the head top surface come into contact with each other is 10 mm or more. The front end of the dolly block on the rail end surface side is located 20 mm to 50 mm away from the rail end surface which is yet to be welded. It is shown that use of this technique can set the longitudinal-direction width having a hardness that is lower than that of the base material by 50 Hv or more to 15 mm or less.
Patent Document 4 shows an example in which the range of the latter flash-off distance is 2 mm to 8 mm, and the range of the latter flashing velocity is 1 mm/s to 4 mm/s when a hot rolling billet is welded using a flash welder with an AC power supply. The welding was performed after the billet is extracted from a billet heating furnace, and before supplied to a first roller.
Here, the latter flashing velocity is (the entire flash-off distance−the former flash-off distance)/(the entire flashing time−the former flashing time).
Patent Document 5 shows a continuous rolling method of a metal material in which the rear end of a preceding material to be welded is joined to the front end of a following material to be welded by flash butt welding, and then the materials are continuously rolled using downstream rolling mills array, thereby producing a metal finishing material, in which the materials are joined with the flashing amount Y (flash-off distance) set to satisfy the following Formula (2).0.1D≦Y<0.30D  Formula (2)
Herein, Y is the flashing amount (flash-off distance) (mm), D is the diameter of the material to be welded (mm), and the flashing amount (flash-off distance) is the total distance of the material that is melted and removed by an arc during the flash butt welding.
Non-patent document 2 shows a method in which the flashing (flash) velocity is abruptly increased immediately before upsetting, and the butt end surface is flattened and smoothened as a method for producing a high-quality flash butt welding joint by flash-butt-welding of a rail. Specifically, it is shown that a favorable final flashing (flash) velocity is 1.0 mm/sec to 1.25 mm/sec. The flash-off distance at this time is stated to be 3 mm.